Field of the Invention
The present invention relates to the field of electrode materials, and more specifically, to an alkali metal oxyanion electrode material having a carbon deposited by pyrolysis, as well as to a process for making same.
Description of Related Art
Positive electrodes of lithium-ion battery generally comprise an electrochemically active cathode material, a binder and carbon particles which act as an electronically conductive additive. During battery cycling, it has been observed that such positive electrodes generally show a deleterious increase in electrode resistance. It has been proposed that as the number of cycles increases, the cathode material exhibits unit lattice volume expansion/shrinkage variations that are due to insertion/deinsertion of alkali cation in the cathode material. It is believed that these variations induce a loss of electronically conductive network contact with the cathode material and/or breaking of cathode material particles. As a result, the battery's capacity decreases and the battery has a resulting shorter life span.
WO 2009/096255 describes a cathode material of general formula LiyKaFe1-xXxPO4 with reduced change of the unit cell volume, where X represents at least one element selected from group 2 to 13 elements; 0<a≦0.25 and 0≦x≦0.25; and y is (1−a). The Li site is partially substituted with at least K to prevent a volume change from occurring due to Li desorption. The percentage change of the unit lattice volume is 4% or less.
WO 2010/134579 describes an alkali metal phosphosilicate cathode material of general formula LiFe1-xMxP1-ySiyO4 where the average valence of Fe is +2 or greater; M is an element having a valence of +2 or greater and is at least one selected from the group consisting of Zr, Sn, Y and Al; the valence of M and the average valence of Fe are different; 0<x≦0.5; and y=x·({valence of M}−2)+(1−x) ({average valence of Fe}−2).
JP 2011/077030 describes an alkali metal phosphosilicate material of general formula Li(1-a)AaFe(1-x-b)M(x-c)P(1-y)SiyO4, where A is at least one kind selected from a group consisting of Na, K, Fe and M. Average valence of Fe is +2 or more, M is an element of valence of +2 or more, and at least one kind selected from the group consisting of Zr, Sn, Y and Al, the average valence of M and the average valence of Fe are different from each other, and where 0<a≦0.125, 0<x≦0.5, and 0<y≦0.5. The volume change of the unit cell is 5% or less.
WO 2012/061934 describes an alkali metal phosphosilicate cathode material comprising particles which carry, on at least a portion of their surface, carbon deposited by pyrolysis, where the particles have a general formula AM1-xM′xP1-ySiyO4 where the average valence of M is +2 or greater; M is Fe and/or Mn; and A is at least one alkali metal selected from Li, Na and K. Optionally, the Fe and/or Mn is substituted by at most 15% at. of one or more metal at oxidation levels between +1 and +5. M′ is a metal of valence of 2+ or more. The x, y and z are defined as follows: 0<x≦0.25; and y=x·({valence of M′}−2)+(1−x)·({average valence of M}−2). WO 2012/061934 also describes a two-step process for making this material. The process includes a first dry high-energy milling step of precursors of the carbon-deposited alkali metal phosphosilicate cathode material which step is performed prior to a first solid-state thermal reaction, and a second dry high-energy milling step on a product of the first thermal reaction which second step is performed prior to a second solid-state thermal reaction.